Hydraulic brake systems require vacuum pressure to efficiently operate the brakes. Vacuum pressure from a combustion engine is typically used to provide the required vacuum for the brake system. However, the vacuum pressure demand by the brake system may occur in situations when the vacuum pressure available from the engine is not sufficient, e.g. cold start or high altitude. Insufficient vacuum pressure within the brake system is particularly common in direct injected engines over those that are naturally aspirated. However, other considerations may make use of a directly injected engine desirable. Therefore, a vacuum pump is used to provide the additional vacuum pressure required by the brake system.
Vacuum pumps can be mounted to the engine and driven by the crank shaft for the engine. However, mounting of the vacuum pump to the engine results in the pump continuously running while the engine is on. This is inefficient as the vacuum pump is running even when the engine is providing sufficient vacuum.
Alternatively, vacuum pumps can be independently driven by a vacuum pump motor. The vacuum pump motor can drive the vacuum pump only when necessary to produce vacuum pressure for the brake system. However, the size of motor required to produce the necessary vacuum pressure in an efficient manner is large and adds cost and weight to the vehicle.
The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.